
4 answers
The formula used in a formula field is considered metadata. To export metadata there are several options, some of which are included here (https://success.salesforce.com/answers?id=90630000000DBNTAA4)
The best and most thorough way would probably be to use the Force.com IDE to export metadata from a particular object. However if you're not familiar with it, another option might be to use an AppExchange app.